This invention relates to determining location in mobile communication systems and particularly though not exclusively, to the cellular GSM system (Global System for Mobile Communications), a description of which can be found in the book xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d by M. Mouley and M. Pautet.
A cellular radio telephone radio system generally includes a switch controller coupled to the public switched telephone network (PSTN) and a plurality of base stations. Each of the plurality of base stations generally defines a geographic region or xe2x80x9ccellxe2x80x9d proximate to the base station to produce coverage areas. One or more mobile stations communicate with a base station that facilitates a call between the mobile station and the PSTN. The communication link over a carrier signal from the base station to a mobile station is referred to as the downlink. Conversely, the communication link from a mobile station to the base station is referred to as the uplink.
GSM networks are made up of mobile services switching centers, base station systems and mobile stations. These three entities can be broken down further into smaller entities such as within the base station system there are base station controllers, base transceiver stations and transcoders.
It may be advantageous to determine the location of a mobile station. For example, such information would be useful to emergency, tariffing services and traffic routing services. The accuracy required is dictated by the service for which location determination is needed. For example, for tariffing, an accuracy of 500 meters might be acceptable. However, for the purpose of locating the nearest restaurant, for example, an accuracy of less than 100 meters would be needed. The initial driving force for location determination was the legislation brought out in the United States of America (known as E911) which requires that a mobile station can be located to within 125 meters for 67% of the time for the emergency services. This is to be introduced in stages and is expected to be operational by the year 2001. At present, in countries operating GSM networks there is no such legislation in place. However, the need for location determination in GSM systems is gaining momentum with a plethora of services being anticipated and also location-selective tariffing being proposed.
Most known methods of locating a mobile station in a mobile telecommunications network involve a calculation of the distances between three or more base stations and mobile stations. One known method of estimating the distance between a base station and a mobile station in a GSM network involves measuring the propagation delay incurred by the carrier signal during its return trip from the base station to a mobile station. (See for example, WO-A-9205672). Once the distance between three or more known fixed points and the mobile stations are known, the mobile station""s relative position can be calculated. For example, the so-called xe2x80x9ccircle methodxe2x80x9d described in WO-A-9205672 is based on measurements of three independent distances. With three given known reference points in a plane, it is possible to establish the co-ordinates of any other point in the same plane using the distances between the reference points and the point in question.
Location of a mobile station by using the known technique based on measurements of propagation delay require that the mobile station is simultaneously visible to at least three base stations. Environmental constraints on the cellular network may not always make this possible.
Thus there is a need for a system and method of location which removes this constraint.
In one aspect, the present invention consists of apparatus for determining the location of a mobile station in a mobile telecommunications system, the apparatus comprising:
a computation device,
a master station, and
a plurality of slave stations positioned at known locations.
The master station is adapted to communicate with the mobile station and with the plurality of slave stations,
Each slave station incorporates synchronization means, a receiver for receiving signals from the mobile station, measuring means for measuring the time of arrival of the signals, and a transmitter for facilitating transmission of the time of arrival measurements to the computation device for calculating the location of the mobile station, said transmission of the time of arrival measurements being at least partly over an uplink channel between the slave stations and the master base station.
The computation device may be located at the master station or at some other remote location. Time of arrival measurements may be transmitted directly to such a remote location or via the master station.
The master station may, in part, comprise a conventional base transceiver station fitted with additional components in accordance with the invention.
The slave stations may comprise a modified form of radio telecommunication handset and may, conveniently, be mounted at the top of lamp posts, for example.
The synchronization means required by each slave station could comprise part of a commercially available GPS receiver (Global Positioning System).
Hence the invention advantageously requires only relatively inexpensive equipment additional to the mobile telecommunication infrastructure. Each base transceiver station in the network would preferably control a minimum of three slave stations (in order to give a reasonably accurate fix on the location of the mobile station) within the cell it serves. Conveniently, one could be co-located with the master station. The slave stations need only be small and can be mounted almost anywhere, preferably at a site with an electrical power supply. They could, of course, be powered by battery or solar cells. Another advantage is that they do not require a dedicated leased telecommunications line.
The invention provides the further advantage in that only one transceiver station needs to be used in the location process. Therefore, in large cells where there may be no or very weak signals from neighbouring base transceivers stations, location estimates can still be performed.
Further, by virtue of this invention, location determination by time of arrival measurements does not require time synchronisation of the entire telecommunications network. This would otherwise have to be done by the additional, complicated method of using a common clock, or establishing a reference station which calculates the time offsets between different components of the network. Using this invention, it is sufficient for just the slave stations peculiar to a cell to be synchronised.
The signals sent by the mobile station to the slave stations can be kept very simple in content, i.e. no large amount of data needs to be transmitted over the network. The signals may comprise conventional xe2x80x9cfill-framesxe2x80x9d for example.
Further, the data generated by the slave stations e.g. time of arrival measurements can be transmitted in SMS format (short message service), GPRS or USSD. This latter facet enables multiple measurements to be made in order to improve the accuracy of the location computation without putting a prohibitive load on the network.
Hence the location apparatus of the present invention can be xe2x80x9coverlaidxe2x80x9d on an existing radio communications network and be invisible to it.
Initial control commands may originate from the master station (i.e. a serving base transceiver station in one particular embodiment) or the location procedure can be initiated either by the mobile station or by the controlling network components. (For example, a mobile switching center).
The apparatus can be extended in the cellular network context so that any slave station can be used by any base transceiver station within range.
In a second aspect, the invention consists of a method for determining the location of a mobile station in a mobile telecommunication system, the method including the steps of:
receiving at a plurality of time-synchronised slave station, control signals from a master station,
receiving at each of the slave stations signals from the mobile station,
measuring at each of the slave stations the time of arrival of the signals, from the mobile station.
transmitting from each of the slave stations the time of arrival measurements to a computation device,
and computing in the computation device the location of the mobile station with respect to the slave stations according to the time of arrival measurements.
The method may further comprise the step, in the master station, of receiving a location request from either the mobile station or from a controlling element of the telecommunications network.
In a preferred embodiment, the master station designates particular slave stations for receiving signals from the mobile station and for performing the time of arrival measurements. It may also instruct the mobile station when, at a particular moment in time, to transmit a signal burst.